ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-13-4563-2013Variability in the speed of the Brewer–Dobson circulation as observed by Aura/MLSFluryT.13WuD. L.2ReadW. G.11Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA2NASA-Goddard Space Flight Center, Greenbelt, Maryland, USA3now at: Swiss Federal Office of Public Health, Bern, Switzerland0205201313945634575This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/13/4563/2013/acp-13-4563-2013.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/4563/2013/acp-13-4563-2013.pdf

We use Aura/MLS stratospheric water vapour (H<sub>2</sub>O) measurements as tracer
for dynamics and infer interannual variations in the speed of the
Brewer–Dobson circulation (BDC) from 2004 to 2011. We correlate one-year
time series of H<sub>2</sub>O in the lower stratosphere at two subsequent pressure
levels (68 hPa, ~18.8 km and 56 hPa, ~19.9 km at the Equator)
and determine the time lag for best correlation. The same calculation is made
on the horizontal on the 100 hPa (~16.6 km) level by correlating the
H<sub>2</sub>O time series at the Equator with the ones at 40° N and
40° S. From these lag coefficients we derive the vertical and
horizontal speeds of the BDC in the tropics and extra-tropics, respectively.
We observe a clear interannual variability of the vertical and horizontal
branch. The variability reflects signatures of the Quasi Biennial Oscillation
(QBO). Our measurements confirm the QBO meridional circulation anomalies and
show that the speed variations in the two branches of the BDC are out of
phase and fairly well anti-correlated. Maximum ascent rates are found during
the QBO easterly phase. We also find that transport of H<sub>2</sub>O towards the
Northern Hemisphere (NH) is on the average two times faster than to the
Southern Hemisphere (SH) with a mean speed of 1.15 m s<sup>−1</sup> at 100 hPa.
Furthermore, the speed towards the NH shows much more interannual variability
with an amplitude of about 21% whilst the speed towards the SH varies by
only 10%. An amplitude of 21% is also observed in the variability of
the ascent rate at the Equator which is on the average 0.2 mm s<sup>−1</sup>.